Enterically transmitted non-A/non-B hepatitis viral agent (ET-NANB; also referred to herein as HEV) is the reported cause of hepatitis in several epidemics and sporadic cases in Asia, Africa, Europe, Mexico, and the Indian subcontinent. Infection is usually by water contaminated with feces, although there is some evidence of person to person transmission. The virus does not seem to cause chronic infection. The viral etiology in ET-NANB has been demonstrated by infection of volunteers with pooled fecal isolates; immune electron microscopy (IEM) studies have shown virus particles with 27-34 nm diameters in stools from infected individuals. The virus particles reacted with antibodies in serum from infected individuals from geographically distinct regions, suggesting that a single viral agent or class is responsible for the majority of ET-NANB hepatitis seen worldwide. No antibody reaction was seen in serum from individuals infected with parenterally transmitted NANB virus (also known as hepatitis C virus or HCV), indicating a different specificity between the two NANB types.
In addition to serological differences, the two types of NANB infection show distinct clinical differences. ET-NANB is characteristically an acute infection, often associated with fever and arthralgia, and with portal inflammation and associated bile stasis in liver biopsy specimens (Arankalle, 1988). Symptoms are usually resolved within six weeks. Parenterally transmitted NANB, by contrast, produces a chronic infection in about 50% of the cases. Fever and arthralgia are rarely seen, and inflammation has a predominantly parenchymal distribution (Khuroo, 1980). The course of ET-NANBH is generally uneventful in healthy individuals, and the vast majority of those infected recover without the chronic sequelae seen with HCV. Occasionally the course of disease can be severe, however, as was recently shown by a human volunteer (Chauhan 1993). One peculiar epidemiologic feature of this disease, however, is the markedly high mortality observed in pregnant women; this is reported in numerous studies to be on the order of 10-20% (Khuroo 1981, Reyes 1993). This finding has been seen in a number of epidemiologic studies but at present remains unexplained. Whether this reflects viral pathogenicity, the lethal consequence of the interaction of virus and immune suppressed (pregnant) host, or a reflection of the debilitated prenatal health of a susceptible malnourished population remains to be clarified.
The two viral agents can also be distinguished on the basis of primate host susceptibility. ET-NANB, but not the parenterally transmitted agent, can be transmitted to cynomolgus monkeys. The parenterally transmitted agent is more readily transmitted to chimpanzees than is ET-NANB (Bradley, 1987).
In the earlier-filed parent applications, HEV clones, peptide antigens, and the sequence of the entire HEV genome sequence were disclosed. From HEV ORF-2 expression constructs, recombinant proteins and viral particles were produced.